1. Field of the Invention
The present invention relates to an abnormality diagnosis device and a control system of a multi-cylinder internal combustion engine having a diagnosing device for diagnosing presence or absence of an abnormality in an individual air-fuel ratio of each of multiple cylinders based upon a sensing value of an air-fuel ratio sensing device arranged downstream of a junction of the engine where exhaust gases discharged respectively from the multiple cylinders converge.
2. Description of Related Art
As an abnormality diagnosis device of this type, there is a proposed abnormality diagnosis device that diagnoses presence or absence of an abnormality in an air-fuel ratio of an arbitrary cylinder by comparing the air-fuel ratio of the arbitrary cylinder with an average value of air-fuel ratios of the other cylinders except for the minimum value and the maximum value of the air-fuel ratio (for example, as described in JP-A-2006-138280). With this abnormality diagnosis device, even when an abnormality arises in the air-fuel ratio of the cylinder other than the arbitrary cylinder, the air-fuel ratio of the abnormal cylinder can be excluded from the comparison object to be compared with the air-fuel ratio of the arbitrary cylinder. As a result, the presence or absence of the abnormality in the air-fuel ratio of the arbitrary cylinder can be diagnosed with high accuracy.
The inventor found that in the case of estimating the air-fuel ratio of each cylinder based upon the sensing value of the air-fuel ratio sensor arranged downstream of the junction, estimation accuracy of the air-fuel ratio deteriorates when an exhaust quantity discharged from each cylinder per exhaust stroke is small. Therefore, the abnormality diagnosis device described above cannot appropriately detect an abnormality even if the abnormality arises in an air-fuel ratio of a certain cylinder when the exhaust quantity from each cylinder is small.